The invention relates to lamellar gratings, and more particularly to lamellar gratings having a substantially polarization-independent diffraction efficiency over a wavelength range useful for optical telecommunication applications.
Fiber optic telecommunication systems are increasingly used to provide high-bandwidth transmission of information to homes and businesses. During the past decade, optical networks have become more complex and use multiple wavelengths transmitted simultaneously over the same fiber. This transmission method is referred to as wavelength division multiplexing/demultiplexing (WDM/D). The international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing (xcx9c0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less. In addition, the total wavelength range over which these devices are designed to operate is increasing. WDM is a general term applied to the separation and integration of information carried on these optical channels. The technologies involved in WDM/D being routed through various devices that deliver the high bandwidth signals to the end customer.
To extract information from WDM channels, the various optical carrier frequencies propagating, for example, in a communication fiber, have to be separated. Wavelength-selective optical elements, such as interference filters, fiber Bragg gratings, arrayed waveguide gratings (AWG), and free space gratings, e.g., surface relief diffraction gratings, are employed for this purpose. Many of these wavelength-selective components have a polarization-sensitive response. The gratings can be ruled gratings, holographic gratings or etched gratings. Gratings may employ a crystalline substrate, such as a Si wafer, that can be processed by conventional semiconductor processing techniques. In particular, grating structures can be formed in Si by preferential etching using an aqueous solution of KOH to expose the Si (111) planes, or by (non-preferential) reactive ion etching which allows the formation of arbitrary grating characteristics.
It is well known that standard single-mode fiber may not preserve the launched state of polarization (SOP) of optical signals propagating through the fiber. Moreover, the SOP usually varies with time at any given point along the fiber due to small changes in the physical environment of the fiber or in the optical wavelengths. These random polarization fluctuations can affect transmission systems that employ polarization-sensitive optical components, such as optical amplifiers, coherent optical receivers or polarization-dependent photonic switches and demultiplexers. Polarization scramblers have recently been employed in optically amplified transoceanic communication systems, where they are used, for example, to eliminate anisotropic gain saturation (polarization hole burning) in the optical amplifiers by depolarizing the launched optical signal. Accordingly, optical components used with optical fibers should be made polarization independent, thereby reducing costs and complexity of the fiber-optic communications system. Moreover, the optical components should be highly efficient to extend the range of data transmission through optical fibers.
It would therefore be desirable to provide compact wavelength-dispersive devices that can separate closely-spaced optical channels with equal efficiency regardless of the polarization direction of the light signal and with high diffraction efficiency.
The invention is directed, among other things, to lamellar immersion gratings with a high dispersion for wavelength-dispersive applications, such as wavelength filtering, wavelength tuning and wavelength multiplexing/demultiplexing for optical communication systems. The gratings are designed to provide high efficiency single mode (or single propagating order) diffraction in both TM and TE polarizations with a diffraction efficiency that is essentially wavelength-independent over a selected communication channel, such as the C-band between 1530 and 1565 nm. The high dispersion design makes possible small standalone mux/demux components and integrated subcomponents that can be easily manufactured.
According to one aspect, the lamellar grating is implemented in Si, which is transparent in the infrared portion of the spectrum of interest. The lamellar structure can be fabricated by standard semiconductor processing techniques, such as wet etching, ion beam etching or reactive ion etching. The grating can be operated in reflection or in transmission. If the grating is operated in reflection, it may be operated either in Littrow, or in non-Littrow, where the light exits the grating at an angle substantially different from the entrance angle. The non-Littrow configuration has advantages in certain applications, for instance when double pass operation of the grating is desired. The lamellar structure may be composed of alternating xe2x80x9cteethxe2x80x9d and grooves which together define a grating period. For example, when using a Si wafer as substrate for the grating, the grooves would be etched, leaving the xe2x80x9cteethxe2x80x9d which are hence formed of the semiconductor material. The grooves can be filled with a dielectric or a metal having a refractive index different from that of the substrate or xe2x80x9cteeth.xe2x80x9d Instead of filling the grooves, the xe2x80x9cteethxe2x80x9d can also be coated with a material having a different dielectric constant or refractive index. It will be understood that the actual diffraction properties of the grating will depend on the dimensions and the refractive indices of the substrate and groove fill material.
The gratings are preferably operated in first order. However, the gratings can also be operated at higher diffraction orders. A grating will typically operate in a low diffraction order if the grating period is comparable to, e.g., within a factor of 2, of the wavelength of the light propagating in the medium in which the grating is formed.
According to another aspect of the invention, a lamellar grating is formed or disposed on a surface of a prism with a high index of refraction, such as Si. The grooves of the grating are filled with a material having a lower index of refraction, such as polymer or glass (nxcx9c1.5). Optionally, a second prism is attached to the grating surface of the first prism. This arrangement has small dimensions due to the large index of the semiconductor (Si) prism and can advantageously be incorporated in an optical connector.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.